The present invention relates to a improvement in a front suspension of the strut type of a wheeled motor vehicle with an engine mounted under a front engine hood such as to provide an arrangement whereby the level of vibrations of the wheeled motor vehicle are reduced. These vibrations are induced by a torque variation produced by the engine at idling due to irregular combustion occurring in the engine. More particularly, the present invention relates to a front suspension arrangement whereby the roll resonance frequency of the vehicle induced by a vibration inducing force applied to the vehicle by the engine is deviated from the frequency of the vibration inducing force, thereby, reducing the level of vibrations of the vehicle when the engine idles.
There is known a front suspension of the strut type as shown in FIG. 4 (refer to Page 22 of SERVICE PERIODICAL NO. 491 III, published by NISSAN MOTOR COMPANY LIMITED on Oct., 1983). This front suspension is used in an automobile powered by a diesel engine.
Referring to FIG. 4, what is generally denoted by the reference numeral 1 is a shock absorber used as a strut. The shock absorber 1 comprises an outer cylindrical column 2 having a lower end linked to a wheel support member, not shown, and a piston rod 3 telescopically received in the outer cylindrical column 2 and projecting upwardly. The piston rod 3 of the shock absorber 1 has an upper end fixedly fit in an inner race 4a of a ball bearing 4 having an outer race 4b fixedly fit in an inner sleeve 5a of a mount insulator 5. The mount insulator 5 comprises the inner sleeve 5a, an outer sleeve 5b that is fixedly secured via a bracket 6 to a vehicle body, not shown, and an elastic body 5c loaded between the inner and outer sleeves 5a and 5b. The elasic body 5c is subject to a shearing stress and elastically deforms in a shearing direction when the inner and outer sleeves 5a and 5b are subject to vibrations or impacts during running of a vehicle. The reference numeral 7 denotes a spring operatively disposed between the outer cylindrical column 2 and the mount insulator 5.
However, with this conventional front suspension, the level of vibrations of the automobile becomes considerably high when the engine is subject to irregular combustion at idling.
This phenomenon taking place in the diesel engine is now described. Since combustion pressure in each of the cylinders of the diesel engine varies with the amount of intake air and amount of fuel injection, there occurs a variation in effective pressure between the cylinders as shown in FIG. 5. This torque variation produced by the engine occurs once in every two crankshaft revolutions of the engine. Thus, the main component of vibration of the engine is of the 0.5 th order component of engine revolution speed. This vibration of the engine E is applied, as a roll input force T, via an engine amount K.sub.M to the vehicle body B, causing the vehicle body B to vibrate about its rolling axis in a .theta. direction as shown in FIG. 7. The frequency range of the 0.5 th order component of the engine vibration is from 5 Hz to 6 Hz when the engine idles at an idling speed falling in a range from 600 to 720 rpm. According to the known suspension, the shock absorber 1 sticks at small vibrations ranging from 5 to 10 Hz. Thus, under this condition, a spring force which the roll input force T is applied to the vehicle body B is determined by the relationship (K.sub.3 &lt;K.sub.1) between a spring constant K.sub.1 of the mount insulator 5 and a vertical spring constant K.sub.3 of the tire and thus approximately equal to K.sub.3, so that a resonance frequency of vibration of the vehicle body B induced by the roll input force T comes into agreement with the frequency of the 0.5 th order component of the engine vibration. This results in an increased level of vibration of the vehicle body (refer to FIGS. 9 and 10) when the engine idles.
Accordingly, an object of the present invention is to reduce the level of vibrations of a wheeled motor vehicle at engine idling.
A specific object of the present invention is to provide a front suspension of the strut type which is improved such that a resonance frequency of vibration of a vehicle body induced by a roll input force does not come into agreement with a frequency of the 0.5 th order component of vibration of an engine when the engine idles.